The present invention concerns a composition, particularly for converting hydrogen sulphide and sulphur dioxide to sulphur, and its use in a process for the treatment of a gaseous effluent containing the composition.
In particular, the invention concerns the treatment of Claus plant tail gases originating from hydrodesulphurization and catalytic cracking units.
The prior art is illustrated in U.S. Pat. Nos. 5,429,684, 4,911,856, 3,676,356, 4,056,606, 2,998,304 and French patent FR-A-2 115 721.
The Claus process is widely used to recover elemental sulphur from gaseous feeds containing hydrogen sulphide (H2S). However, the fumes emitted by Claus plant type units contain non negligible quantities of acid gases, even after several catalytic stages. These tail gases from Claus plants must therefore be treated to eliminate the majority of the toxic compounds in order to satisfy anti-pollution regulations. These regulations are becoming more and more strict and existing technology must be constantly improved upon.
As an example, about 95% by weight of the sulphur can be recovered from a Claus plant; treatment of the Claus plant tail gas (using a Clauspol unit, for example), can, for example, recover 99.8% of the sulphur by using the reaction:
2H2S+SO2xc2x73S+2H2O
using a reaction medium constituted by an organic solvent and a catalyst comprising an alkali or alkaline-earth salt of an organic acid. The reaction is advantageously carried out in counter-current mode in a reactor-contractor and the temperature is controlled by passing solvent extracted from the lower extremity of the reactor by means of a circulating pump through a heat exchanger to encourage the highest possible degree of conversion of sulphur, while avoiding the formation of solid sulphur. Sulphur is thus recovered in liquid form. While the process is highly effectively, it is limited by various disadvantages linked to the solvent which, over a period of time which varies depending on the quantity of H2S treated, begins to be oxidised by traces of oxygen contained in the gas.
It has been shown that such oxidation of the solvent causes problems in the operation of the plant:
excessive consumption of catalyst to maintain the catalytic activity of the reaction medium;
poor sulphur decantation;
recovery of poor quality sulphur.
Oxidation appears to be primarily due to traces of oxygen contained in the gas to be treated, but especially due to the presence of trace metals, in particular traces of ferrous or ferric ions, mainly originating from corrosion, which can catalyse the oxidation of the solvent or of the catalytic system.
The invention thus aims to provide a catalytic composition which can overcome these disadvantages and ensure good conversion of H2S and SO2 to sulphur.
More precisely, the composition comprises:
an organic solvent A with a boiling point of more than 200xc2x0 C. at atmospheric pressure;
a mixture B of at least one alkali or alkaline-earth salt (S) of an organic monoacid or an organic polyacid which acts as a catalyst, wherein at least one dissociation constant value (pK) is in the range 2.2 to 8; at least one complexing agent (C); and water (W), in the following proportions by weight;
S=0.1% to 30%;
C=0.001% to 30%;
W=qs 100
The alkali ion is preferably selected from the lithium, sodium, potassium or ammonium ion and is advantageously provided in the form of the hydroxide.
In general, an excess of alkali or alkaline-earth ions is used, for example an ion/organic acid ratio which is in the range 1 to 10, preferably in the range 2 to 7.
Advantageously, the catalyst (organic acid salt) has at least one dissociation constant (pK) value in the range 2.6 to 6.
Preferably, the best conversion results are obtained when the catalyst is selected from the group formed by formic acid, acetic acid, ascorbic acid, fumaric acid, maleic acid, malonic acid, oxalic acid, tartaric acid, benzoic acid, salicylic acid and sulphosalicylic acid.
In the steady state, it is used at a concentration of 1% to 5% by weight, preferably 0.5% to 2% by weight with respect to the mixture.
In accordance with the process, the complexing agent is at least one alkaline or alkaline-earth salt or a ferrous or ferric salt of a mono- or poly-aminocarboxylic acid, citric acid, salicylic acid or sulphosalicylic acid, a sulphocyanide ion, a ferrocyanide ion, a ferricyanide ion, a phosphate ion, a pyrophosphate ion, a fluoride ion and/or a thiosulphate ion.
The term xe2x80x9cmono- or poly-aminocarboxylic acidxe2x80x9d means a nitrogen-containing compound containing 4 to 20 carbon atoms, preferably:
nitrilotriacetic acid, NTA;
ethylene diamine tetraacetic acid, EDTA;
hydroxyethylene diamine tetraacetic acid, HEDTA; and
imidodiacetic acid.
Excellent results have been obtained when the concentrations of complexing agent (or chelating agent) in mixture B are advantageously in the range 0.1% to 5%, preferably in the range 0.01% to 1% by weight.
In a further characteristic of the process, the mixture also contains 0.01% to 10% by weight of at least one anti-oxidant, preferably 0.1% to 2% by weight. Anti-oxidants are generally soluble in the mixture and the organic solvent and are normally selected from propyl gallate, ter-butylhydroquinone, 2,3-534-butyl-4-hydroxyanisole, 3,5-diter-butyl-4-hydroxytoluene, 2,6-di-ter-butyl-4-methylphenol (trade name: Ionol), octyl gallate, 2,4,5,-trihydroxy-butyrophenone, nordihydroguaiaretic acid, 2,6-di-ter-butyl-4-hydroxyrnethylphenol, lauryl gallate, 6-ethoxy-1,2-dihydro-2,2,4-trimethylquinoline (ethoxyquin); and the thiosulphate, chloride formate and oxalate of alkali salts (in particular sodium, potassium and ammonium).
When the anti-oxidant in the composition is organic, its concentration in the mixture is preferably in the range 200 to 1000 ppm by weight; when it is associated with an alkali ion, its concentration is normally higher, for example 0.2% to 1% by weight with respect to the mixture.
In a further characteristic of the process, mixture B can also contain 5 ppm to 1000 ppm of at least one wetting agent which encourages separation of sulphur, preferably 10 to 500 ppm by weight.
The wetting agent or surfactant is advantageously selected from the group formed by sodium dioctylsulphosuccinate, sodium dihexylsulphosuccinate, and sodium diamylsulphosuccinate; more particularly, with a concentration of these products in the range 100 to 200 ppm by weight, excellent results were obtained.
In a further characteristic of the process, it is generally useful to introduce 5 to 10000 ppm by weight of at least one anti-foaming agent, preferably 10 to 500 ppm by weight with respect to mixture B. The most advantageous products can be an alkylene diol-organosilane, for example a product prepared from di-(2-hydroxy-n-propyl) ether and methyltrichlorosilane, or a polyalkylene methylpolysiloxane, or a silicone-glycol copolymer, or an oxyalkylated alcohol.
Solvent A of the composition can be selected from solvents which are insoluble in water or from solvents which are soluble in water, water being one of the products of the reaction of SO2 and H2S:
Of solvents which are insoluble in water, hydrocarbons with boiling points of more than 250xc2x0 C. at atmospheric pressure are used, preferably dodecane, tridecane, or naphtha with boiling points which are, for example, in the range 225xc2x0 C. to 335xc2x0 C.
Of solvents which are soluble in water with boiling points at atmospheric pressure of more than 200xc2x0 C., polyols containing 3 to 15 carbon atoms are used, preferably glycerol, thiodiglycol and cyclohexane dimethyl ethanol, esters of organic acids containing 5 to 15 carbon atoms, which may be hydroxylated, more particularly trimethyl-pentane-diol-monoisobutyrate and dimethyl-adipate, glycol ethers containing 5 to 15 carbon atoms, advantageously butoxytriglycol, ethoxytriglycol, diethylene glycol butyl ether, ethylene glycol phenyl ether, ter-phenyl ethylene glycol monobenzyl ether, ethylene glycol butyl-phenyl ether, diethylene glycol dibutyl ether, tetra-ethylene glycol dimethyl ether; propylene n-butyl ether, dipropylene n-butyl ether; tripropylene n-butyl ether; diethylene glycol, triethylene glycol and polyethylene glycol with a molecular mass of 200, 300, 400 or 600.
In general in the composition, mixture B represents 0.001% to 10% by weight of solvent A. It is advantageously in a proportion corresponding to a value which is in the range 100 ppm to 5000 ppm, preferably in the range 200 to 1000 ppm (parts per million).
The invention also concerns the use of the composition, in particular in a process for eliminating hydrogen sulphide and sulphur dioxide contained in a gaseous effluent.
The composition is normally used in a proportion of 1% to 40% by volume with respect to the sulphur-containing gaseous effluent, preferably in a proportion of 3% to 30% by volume.   ·            (              liquid        ⁢                  xe2x80x83                ⁢        volume        ⁢                  xe2x80x83                ⁢        of        ⁢                  xe2x80x83                ⁢        composition        xc3x97        100            )              (                        volume          ⁢                      xe2x80x83                    ⁢          of          ⁢                      xe2x80x83                    ⁢          gas                +                  liquid          ⁢                      xe2x80x83                    ⁢          volume          ⁢                      xe2x80x83                    ⁢          of          ⁢                      xe2x80x83                    ⁢          composition                    )      
It is normally brought into contact with this effluent at a temperature which is in the range 20xc2x0 C. to 150xc2x0 C., preferably in the range 60xc2x0 C. to 130xc2x0 C.
The advantages of the composition and its use in a process for treating a gaseous effluent containing H2S and SO2 will become clear from the following examples illustrated by the accompanying FIGURE which shows an implementation of the process in a conventional apparatus.